The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen(77 K) and room temperature by using the direct current heating method.The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values.However,the electrical conductivity drop is considerably greater than the thermal conductivity drop,which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport,hence leading to the violation of the Wiedemann-Pranz law.We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity.Moreover,a modified Wiedemann-Pranz law is provided in this paper,the obtained results from which are in good agreement with the experimental data.
